Power amplifiers are electronic devices operable for amplifying an input to a level that is suitable for driving a load, such as an audio speaker, a transducer, or an electric motor. For alternating current (AC) input signals, the impedance of the load is determined by the frequency of the input signal as well as the resistance, capacitance, and inductance of the load. Power amplifiers typically include a power supply, an input stage, and an output stage. The power supply may be of the linear type or the switching mode type, with the latter type providing higher relative energy efficiencies. There is a particular type of power amplifier known as a switchmode power amplifier. This type of power amplifier may also contain a power supply of the linear type or of the switching mode type.
A switch mode power supply may also be used to modulate a signal in a particular type of switchmode power amplifier. The output stage, controlled by the input stage, applies precisely timed pulses to a load to amplify the relatively weak input signal and thereby generate an output signal having a power level sufficient for driving the load. Such power amplification may be provided via a transformer. Conventionally, the power supply remains connected to the load, with the impedance of the load tending to lower the overall efficiency of the amplifier.
Of the power amplifier types noted above, switching mode-type power amplifiers in particular operate by applying electrical power to the load. For instance, fixed or variable width pulses representing a desired signal may be provided by a fixed-amplitude power supply at precise intervals. Alternatively, fixed or variable width pulses may be provided from a variable power supply. The frequency of the switching pulses is significantly higher than that of the desired output signal so that energy from the switching pulses can be integrated over time to reproduce the desired signal. A high switching frequency is also desirable in order to simplify the task of filtering out undesired energy produced at the switching frequency. Depending on the impedance of the load, higher power required at the load may require a higher voltage. Thus, the power supply used for high-impedance loads must produce higher voltage levels relative to voltage levels used with low impedance loads.
The switching speed of a typical solid-state semiconductor switch operating at higher voltage levels is relatively slow compared to the speed of a switch used in lower voltage devices. The parasitic series loss of a high-voltage semiconductor switch is also higher than a lower voltage device. As a result, it may be difficult to precisely time the delivery of signal pulses to a given load. Precisely timed delivery of signal pulses is important to maintaining high signal fidelity. If the load is connected during the switching interval, the finite switching time and parasitic switching loss of the semiconductor switch will result in increased distortion observable at the load.
A switching mode amplifier may employ a boost transformer. If the boost transformer also carries the desired signal in addition to the switching signal, the boost transformer design is limited by the relationship to the frequency of the carrier signal, i.e., the carrier frequency. Prior art designs without load isolation require a close relationship between the signal frequency and the magnetic design. For instance, if the demodulated signal frequency falls within an example frequency range used for driving audio applications, the magnetizing inductance must be high with respect to the demodulated frequency involved and therefore the transformer must use a magnetic core having a high magnetic permeability in order to be sufficiently compact for practical use. Furthermore, the saturation profile of any magnetic material used in the construction of the transformer is directly related to the demodulated frequency or carrier frequency. This relationship to the demodulated or carrier signal significantly limits the types of magnetic material that can be used, and also limits the choice of upper switching frequency. This limit on upper switching frequency is due to accumulation of eddy current losses and other factors.
Modulation techniques such as pulse width modulation (PWM) or delta-sigma modulation (DSM) may be used for the carrier function in an amplifier. Delta-sigma modulators, which convert a high-resolution input signal into a high-frequency signal having a relatively low resolution, e.g., a 1-bit pulse train, are particularly useful when the ratio of modulation to the carrier signal is relatively low, for instance a ratio of less than 10. DSM can be used to shape quantization noise and thereby reduce noise within the frequency range of the input signal. PWM can be used when the ratio of modulation to the carrier is relatively large, e.g., greater than 10. PWM may be easier to implement for the case of higher ratio of modulation-to-signal, but requires a faster switching speed than DSM. The present state of the art attempts to minimize the effects of finite transition time of the transistors used in the design of power amplifiers of the types using high-speed switching.